A projection type image display apparatus such as an LCD projector occasionally comprises a lens shift mechanism in its projection optical system. The lens shift mechanism is a mechanism that moves a projection lens so as to shift the projection lens in vertical and horizontal directions which are directions orthogonal to an optical axis in order to adjust the image display position on the screen. With a lens shift function using this lens shift mechanism, a degree of freedom on installing the projection type image display apparatus can be increased.
In a manual-type lens shift mechanism, a knob or the like is provided on a housing of the apparatus as a manual operation portion. A member or the like for converting a rotational force into a vertical or horizontal force and transmitting its force is provided with respect to a rotation shaft of the knob. When the knob is turned, the projection lens moves in the vertical or horizontal direction through this mechanism.
Japanese Patent Application Laid-Open Publication No. 2010-256388 (Patent Document 1) is cited as an example of a prior art in regard to a lens shift mechanism. Patent Document 1 has described a lens shift apparatus of an optical device in which miniaturization is achieved by the use of common members. This lens shift apparatus comprises a manual operation portion, a guide mechanism portion, and a motion transmitting mechanism portion. The motion transmitting mechanism portion converts a shift in a rotational action of the manual operation portion into a movement, so that a lens support portion shifts in the horizontal or vertical direction along the guide mechanism portion. The motion transmitting mechanism portion comprises a clutch mechanism portion that cuts off the transmitting mechanism when a rotational torque of the manual operation portion exceeds a predetermined value, thereby causing the manual operation portion to idly rotate.